Fuse position

Hi,

In wiring up a bow thruster, i have a set of batteries located at the bow with the thruster (it is a Vetus 160kgf). The batteries are series, two identical N200 (200Ah batteries).

The power for charging comes from the lazarette of this boat, it is a 30A ac/dc standard tyle charger, 24VDC.

I have a fuse in the line from the batteries to the bow thruster, at the batteries, also isolation switch and isolation switch from the battery charger to the batteries at the batteries (convenience whilst there to turn off charger)

My question please, the long line (8B&S) from the charger to the batteries is unfused at present (Wrong!), so I want to fuse it. Do I put the fuse at the charger or the battery end. The wires are about 15m, so it is quite long and needs to be protected.

Traditionally the fuse is places as close to the power source as possible. If the circuit breaker is turned off at the batteries (for the charger line), then there is no feed back to the charger, though the line is still active of course because the charger is on, if I put the fuse at the charger end, the line could still be active back from the batteries if the isolator switch was left closed.

I believe I need two fuses, one each end of the charging line.

Your thoughts please.

 

I would think that the greater danger is the batteries since they can deliver far more current than the charger and do it catastrophically. The charger should be protected anyway one way or another internally if you short the output ??

 

Both ends. It won't cost much more, and everything in between will be protected no matter the source of the overload.

 

Remove the fuse between the batteries and the bow thruster unless it is a special device for this circuit ( approx 80 Amps). Anything else will blow the first time you use the thruster.
Install a 30 Amp fuse near the batteries to be charged. And of course you have used well insulated wires and terminals that can handle the current.

 

Slightly off topic but...

The biggest problem with a battery charger is when you accidentally connect the pos and neg up the wrong way, usually at night when it is most important to do

There is an easy way to keep this from happening, and will save the charger many times over.

You need a diode and a suitable relay that can switch the charger current.

The charger pos goes to a normally open contact
The contact wiper goes to the battery to be charged
The diode anode also goes to the contact wiper and the cathode goes to the relay coil.
The other coil wire connects to the negative.

It works like this -
If the battery charger is connected to the charger correctly then the battery excites the relay coil through the diode and the contact closes, the charger current now flows through the contact and the battery charges.

If the battery gets connected the wrong way round, the diode polarity blocks the dc and the relay coil cannot excite, the contact doesn't close and the charger doesn't blow up.

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